Currently, many types of optical discs such as DVD and Blu-ray (registered trademark) disc (hereinafter referred to as BD) are used as an information recording medium that stores video, data, or the like. These optical discs are higher in storage reliability compared to a hard disk device (hereinafter referred to as HDD) or magnetic tape. Therefore, the application of optical disc is expanding from the conventional application of recording audio video (AV) data such as video or sound to application of long-term storage of data.
However, the volume of data that can be stored per volume of an optical disc is only approximately one third compared to that of an HDD or magnetic tape. Therefore, in terms of space efficiency at the time of storage, technical development to improve the volume of data that can be stored per volume without increasing the cost of an optical disc has been desired, and intensive research and development has been ongoing. Recently, out of BDs, BDXL (with a recording density of about 33.4 GB per layer) has been on the market as an optical disc with the highest recording density.
The storage reliability of these optical discs is 50 years or longer. In terms of long-term storage of data, the storage reliability is 10 times or greater compared to the life of approximately five years for an HDD. Therefore, by migrating data for long-term storage from an HDD to an optical disc, it is possible to achieve both long-term storage reliability and reduction in storage cost. Particularly, in contrast to an HDD that consumes electric power at the time of data storage, an optical disc that does not require electric power at the time of data storage can reduce the amount of carbon-dioxide emissions as a green storage and leads to the reduction of power consumption at data centers that has become a major issue in recent years.
However, even with BDXL with the highest recording density among optical discs, the volume of data that can be stored per volume is approximately one third that of HDD. Therefore, the required storage space for an optical disc at the time of data storage is greater than for an HDD, and an improvement in recording density per volume of an optical disc has been desired particularly for applications where the cost requirements with respect to storage space are demanding such as in data centers.
As a technique for improving the recording density per volume of an optical disc, there is a land (inter-groove) groove recording and reproducing technique that can improve the recording density of a track. This is a technique used in DVD-RAMs that improve the recording density of a track by recording data that has conventionally been recorded only in groove or land in both groove and land.
Normally, when the recording density of a track of an optical disc is improved, diffracted light from a groove that is necessary for trace control of a groove as a track by an optical beam decreases, and the optical beam cannot trace the track. When an interval L for grooves or lands is less than λ/NA×0.6 where λ is the wavelength of an optical beam with which an optical disc is irradiated and NA is the numerical aperture of a lens that forms the optical beam, diffracted light from a groove cannot be detected, and control for tracing a track is impossible. With a DVD in which the numerical aperture NA is 0.6 and the wavelength λ of an optical beam is 650 nm, the limit of the track interval L with which diffracted light is detectable is 650 nm. In a DVD-RAM, the track density is improved through realization of a track pitch of 615 nm by recording data in both land and groove (for example, see Patent Literature 1).
With such an optical disc that records data in both land and groove, there needs to be a special measure regarding an address recorded on the optical disc particularly in order to access the position in which data is recorded. This is because addresses need to be arranged with high density in order for data to be recorded in both land and groove. As a conventional address arrangement technique, there is complementary allocated pit address (CAPA) used in DVD-RAMs (for example, see Patent Literature 2) or a technique of causing a wobble only at a groove wall on one side to record address information.
Also, there is a method used in BDs in which only groove is used as a recording track (for example, see Patent Literature 3). Using FIG. 31, the relationship of the address structure and the recording data structure for a BD with a wobble in a track will be described briefly. FIG. 31 is a diagram for illustrating the format of a different conventional optical disc.
In FIG. 31, a recording track 1502 is formed by a groove on an optical disc 1501. In a data recording region 1503, data is recorded. In address information regions 1504, 1505, and 1506, address information for accessing the data recording region 1503 is recorded. The address information is arranged in the same region as recording data. The recording data is superimposed and recorded on the address information. One piece of recording data is recorded in a region configured of three pieces of address information AD1 (Z05), AD2 (Z06), and AD3 (Z07), and a region configured of the three pieces of address information is the data recording region 1503 that is a recording unit for data. An integral multiple of the length of the data recording region 1503 configured of the three pieces of address information does not match the length of the circumference of a track. Therefore, as shown in FIG. 31, the positions of the data recording region 1503 on the circumference are arranged with a displacement between adjacent recording tracks for every circumference of the optical disc.
In the recording track 1502, one bit of the address information AD1, AD2, or AD3 is recorded by partially changing the waveform of a groove with a wobble of certain cycles. A region 1507 shown in enlargement in the lower section of FIG. 31 is a portion corresponding to an address bit that is subjected to modulation called minimum-shift keying (MSK). Since an integral multiple of the wobble cycle and the length of one circumference of a recording track do not match as shown in the lower section of FIG. 31, the phase of a wobble differs by a certain amount between adjacent recording tracks.
In an optical disc configured in this manner, the position of a track in which data is to be recorded is identified to start recording of data or the position of a track in which data has been recorded is identified to start reproduction of data, with the address information AD1, AD2, or AD3 as the reference.
Using FIG. 32, a configuration example of an information recording and reproducing apparatus that achieves recording and reproduction of data with respect to an optical disc shown in FIG. 31 will be described. FIG. 32 is a diagram showing the configuration of the conventional information recording and reproducing apparatus.
In FIG. 32, an optical disc 101 includes a track with a wobble as shown in FIG. 31. On the track, information is recorded. An optical head 103 irradiates the optical disc 101 with an optical beam and outputs an electrical signal according to the amount of reflected light from the optical disc 101. A photodetector of the optical head 103 generates a wobble signal, a data signal, and a servo error signal. The photodetector will be described later. A spindle motor 102 causes the optical disc 101 to spin. Based on a servo error signal, a servo controller 104 controls the position in which the optical head 103 irradiates a track of the optical disc 101 with an optical beam and the rotation speed of the spindle motor 102.
With respect to a data signal from the optical head 103, an analog processing unit 105 performs predetermined high-pass filter (HPF) processing in which DC fluctuation is reduced, low-pass filter (LPF) processing in which high-pass noise unnecessary for data reproduction is removed, automatic gain control (AGC) processing in which amplitude fluctuation of a data signal is reduced, and AD conversion processing in which an analog signal is converted to a digital signal using a clock signal supplied from a data phase-locked loop (PLL) circuit 106. From a data signal processed in the analog processing unit 105, the data PLL circuit 106 generates a clock signal in synchronization with a reproduction signal.
An adaptive equalization filter 107 is configured of, for example, a finite impulse response (FIR) filter and adaptively updates the coefficient of a filter such that a data signal processed in the analog processing unit 105 is provided with intended partial response (PR) characteristics. A data decoder 108 decodes the output of the adaptive equalization filter 107 to binary digital data. Although not shown in the drawing, recorded data is reproduced by performing demodulation processing and error correction processing with respect to a result of decoding by the data decoder 108. For a PR method, it suffices to select an optimum method depending on the recording code and the track recording density. As the PR method, there is the PR1221 method or PR 12221 method, for example.
A PR equalization error detector 109 generates a PR equalization error signal from the difference of an intended PR expected value waveform generated from binary digital data decoded by the data decoder 108 and the output waveform of the adaptive equalization filter 107. The adaptive equalization filter 107 changes the coefficient of a filter such that the PR equalization error signal generated by the PR equalization error detector 109 is reduced.
With respect to a wobble signal from the optical head 103, an analog processing unit 111 performs predetermined HPF processing in which DC fluctuation is reduced, LPF processing in which high-pass noise unnecessary for reproduction of the wobble signal is removed, AGC processing in which amplitude fluctuation of the wobble signal is reduced, and AD conversion processing in which an analog signal is converted to a digital signal using a clock signal supplied from a wobble PLL circuit 113. A band-pass filter (BPF) 112 extracts a signal in a predetermined frequency band from the wobble signal. The wobble PLL circuit 113 generates a clock signal in synchronization with the wobble signal from the wobble signal processed by the BPF 112. An address demodulator 114 demodulates address information from a wobble signal sampled with the clock signal generated by the wobble PLL circuit 113 as the reference.
A system controller 115 performs overall control of respective blocks and controls communication with a host. A recording data modulator 116 modulates user data into a recording data pattern that can be recorded in the optical disc 101. With a laser driver 117, the recording data pattern that has been modulated by the recording data modulator 116 is converted to a light pulse for forming a mark accurately on the optical disc 101 and output to the optical head 103. A laser light source of the optical head 103 emits laser light according to the light pulse. A host interface (I/F) 118 performs exchange of recording data and reproduction data with a host.
Using FIG. 33 and FIG. 34, the data signal and the wobble signal generated by the photodetector embedded in the optical head 103 shown in FIG. 32 will be described.
FIG. 33 is a diagram showing a laser irradiation spot scanning a recording track. In FIG. 33, a recording mark 1704 and a space 1705 are formed on three recording tracks 1701, 1702, and 1703, and a laser irradiation spot 1706 is scanning along the direction of the arrow on the recording track 1702 in the middle.
FIG. 34 is a diagram showing the configuration of a conventional photodetector 1800 for reproducing recording data. The photodetector 1800 includes four-divided light-receiving sections 1801, 1802, 1803, and 1804, amplifiers 1805, 1806, 1807, and 1808 that amplify an output signal from the light-receiving sections 1801, 1802, 1803, and 1804, and an adder 1809 that adds all of an A signal, a B signal, a C signal, and a D signal output from the amplifiers 1805, 1806, 1807, and 1808. Based on the output from the adder 1809, a reproduction data signal is generated.
Although not shown in the drawing, a wobble signal that is a reproduction signal of wobble data of a track is detected in the light-receiving sections 1801, 1802, 1803, and 1804 of the photodetector 1800 as a balance signal for the left and right with respect to the track scanning direction. Therefore, a wobble signal is detected not by adding all of the A signal, the B signal, the C signal, and the D signal output from the four amplifiers 1805, 1806, 1807, and 1808, but by subtracting the C signal from the amplifier 1807 and the D signal from the amplifier 1808 from an added value of the A signal from the amplifier 1805 and the B signal from the amplifier 1806. By causing the recording track as shown in FIG. 33 to be irradiated with laser light and scanned with the laser light in the arrow direction shown in the drawing and receiving reflected light with the photodetector as shown in FIG. 34, a data signal and a wobble signal are reproduced.
Next, using FIG. 32, an example of a recording operation of the information recording and reproducing apparatus in which data is recorded with respect to the optical disc shown in FIG. 31 will be described. The host I/F 118 receives a recording request, recording data, and a logical address from the host. The system controller 115 starts the recording operation of the information recording and reproducing apparatus. The system controller 115 converts the logical address to a physical address on the optical disc 101 and controls the spindle motor 102 and the servo controller 104 to move the optical head 103 to the vicinity of a designated address. The address demodulator 114 demodulates physical address information of the vicinity the designated address from a wobble signal. The system controller 115 checks the position of the optical head 103 based on the physical address information demodulated by the address demodulator 114.
The system controller 115 calculates the difference of the demodulated physical address and the designated address and moves the optical head 103 through a track jump. The system controller 115 causes a track jump to an address slightly before the designated address so that recording can be started from the designated address, and moves the optical head 103 along a track up to the designated address in that state to start the recording from the designated address. The system controller 115 causes the recording data modulator 116 to modulate the recording data from the host, sets the optimum recording power and recording pulse information in the laser driver 117, causes laser to be emitted from the designated address position to start recording, and executes recording of designated recording data.
Next, using FIG. 32, an example of a reproduction operation of the information recording and reproducing apparatus in which data is reproduced with respect to the optical disc shown in FIG. 31 will be described. The host I/F 118 receives a reproduction request and a logical address from the host. The system controller 115 starts the reproduction operation of the information recording and reproducing apparatus. The system controller 115 converts the logical address to a physical address on the optical disc 101 and controls the spindle motor 102 and the servo controller 104 to move the optical head 103 to the vicinity of a designated address. The address demodulator 114 demodulates physical address information of the vicinity the designated address from. The system controller 115 checks the position of the optical head 103 based on the physical address information demodulated by the address demodulator 114. In the case where address information superimposed on recorded data is reproduced by the data decoder 108 at this time, the address information reproduced by the data decoder 108 may be the reference.
The system controller 115 calculates the difference of the demodulated physical address and the designated address and moves the optical head 103 through a track jump. The system controller 115 causes a track jump to an address slightly before the designated address so that reproduction can be started from the designated address, and moves the optical head 103 along a track up to the designated address in that state to start the reproduction from the designated address. The system controller 115 processes a data signal in the analog processing unit 105, the adaptive equalization filter 107, and the data decoder 108, reproduces recording data, and transfers reproduction data to the host via the host I/F 118.
In the case where a beam spot diameter of read laser light at the time of reproduction is not sufficiently small, reducing the track interval in order to improve the recording density per volume as described above increases leakage (crosstalk) of a signal from an adjacent track. In the case where a recorded signal is reproduced, there is a problem that the reproduction quality deteriorates.
In order to solve this problem, Patent Literature 4, for example, discloses a technique in which a memory or delay element is used in a constant angular velocity (CAV) method such that reproduction signals of three tracks that are in synchronization in the radial direction of an optical disc (i.e., a reproduction signal of a reproduction track and a reproduction signal of a track adjacent to the reproduction track) are multiplied by an appropriate coefficient and added to reduce crosstalk between tracks.
In Patent Literature 5, a light-receiving region of a photodetector is divided into three with respect to the direction of scanning by an optical spot on a recording track. Reflected light from a recording track irradiated with the optical spot is received by a main light-receiving region, and reflected light from a track adjacent to the recording track is received by two sub light-receiving regions. A signal processing unit performs waveform equalization of an output signal from the main light-receiving region so as to not cause correlation with an output signal from the sub light-receiving region. Since the output signal from the main light-receiving region is not interfered by the output signal from the sub light-receiving region as a result, the influence of crosstalk can be removed.
In Patent Literature 6, a data detection apparatus includes a plurality of adaptive equalizer units in order to perform crosstalk cancel signal processing ((1) synchronization of reproduction signals of adjacent tracks with channel clock precision and (2) reproduction of frequency characteristics of crosstalk from an adjacent track to a main reproduction track). As a reproduction information signal read from a recording medium, a reproduction information signal from a target track that is a data detection target and a reproduction information signal from a neighboring track near the target track that is a crosstalk component with respect to the reproduction information signal are respectively input to the respective adaptive equalizer units.
The data detection apparatus includes a multi-input adaptive equalizer unit that outputs an equalization signal through operation of the output of each adaptive equalizer unit, a binarization unit that performs binarization processing for the equalization signal output from the multi-input adaptive equalizer unit to obtain binary data, and an equalization error arithmetic unit that obtains an equalization error from an equalization target signal obtained based on a binary detection result of the binarization unit and the equalization signal output from the multi-input adaptive equalizer unit and supplies the equalization error as a tap coefficient control signal for adaptive equalization to each adaptive equalizer unit.
The data detection apparatus includes a memory unit that stores a reproduction information signal read from a recording medium. With a memory controller, a reproduction information signal from a target track and a reproduction information signal from a neighboring track are read at each time point from the memory unit and supplied to each of the plurality of equalizer units. The data detection apparatus further includes a phase difference detection unit that detects the phase difference of respective reproduction information signals read from the memory unit and input to a plurality of the adaptive equalizer units and outputs a correction signal for correction of a read timing of each reproduction information signal from the memory unit based on the detected phase difference.
The multi-input adaptive equalizer unit includes three adaptive equalizer units. The three respective adaptive equalizer units are each input with a reproduction information signal from a target track, a reproduction information signal from a neighboring track adjacent on one side of the target track, and a reproduction information signal from a neighboring track adjacent on the other side of the target track. The multi-input adaptive equalizer unit performs partial response equalization processing for the reproduction information signal from the target track. The binarization unit performs maximum-likelihood decoding processing as the binarization processing for the equalization signal of the multi-input adaptive equalizer unit. The equalization error arithmetic unit obtains the equalization error by operation using the equalization target signal obtained through convolution processing with a binary detection result from maximum-likelihood decoding and the equalization signal output from the multi-input adaptive equalizer unit.
When the track pitch is narrowed in order to improve the recording capacity, a reproduction signal at the time of target track reproduction deteriorates due to crosstalk from an adjacent track. The reproduction signal includes a reproduction signal (RF signal) in which recorded information is reproduced and an address signal for which a wobble is caused in a track with a predetermined method and added as address information.
In order to solve the crosstalk problem with respect to an RF signal, crosstalk cancel signal processing has been proposed (for example, see Patent Literature 4, Patent Literature 5, and Patent Literature 6). The point in performance improvement of the crosstalk cancel signal processing is cancel processing in consideration of (1) synchronization of reproduction signals of adjacent tracks with channel clock precision and (2) reproduction of the frequency characteristics of crosstalk that influences a main reproduction track from an adjacent track. This is because a sufficient performance improvement cannot be obtained with simple subtraction processing since the crosstalk amount from an adjacent track differs depending on the recording mark length.
With the crosstalk cancel signal processing proposed in Patent Literature 4, realization of synchronization of reproduction signals of adjacent tracks described above in (1) can presumably be achieved relatively easily, since the CAV recording method is assumed. However, with this recording method, the recording capacity cannot be improved.
With the crosstalk cancel signal processing proposed in Patent Literature 5, a reproduction signal recorded in a target track and a crosstalk signal from an adjacent track can be detected simultaneously, since a photodetector in which the light-receiving region is divided into three with respect to the direction of scanning an optical spot on a recording track is used. Therefore, with Patent Literature 5, the problem of synchronization of reproduction signals of adjacent tracks described above in (1) does not occur. However, with Patent Literature 5, a sufficient crosstalk cancelling effect may not be obtained since (2) described above is not taken into consideration.
The crosstalk cancel signal processing proposed in Patent Literature 6 is cancel processing in consideration of (1) synchronization of reproduction signals of adjacent tracks with channel clock precision and (2) reproduction of the frequency characteristics of crosstalk that influences a main reproduction track from an adjacent track. In order to perform the synchronization of reproduction signals of adjacent tracks described above in (1), reproduction signals of adjacent tracks are held in a memory at a predetermined timing in Patent Literature 6. Due to such a configuration, there are roughly four problems below in Patent Literature 6.
Problem 1: In order to remove the influence of an adjacent track, a reproduction signal of a reproduction track and a reproduction signal of the adjacent track are necessary. Therefore, at the time of a first read, crosstalk cancel processing cannot be carried out until the reproduction signal of the adjacent track is held in a memory, and the reproduction performance stays deteriorated. That is, with Patent Literature 6, it is always impossible to obtain the effect of the crosstalk cancel processing.
Problem 2: Since a reproduction signal of an adjacent track needs to be secured in a memory, the amount of information that needs to be secured in a memory increases toward the outer circumference side of an optical disc. This leads to an increased circuit scale.
Problem 3: In the case of an optical disc having a double spiral configuration as an optical disc in which data is record in both land and groove instead of an optical disc having a single spiral configuration provided with a CAPA address in an intermediate section between land and groove as an optical disc in which data is record in both land and groove such as a RAM disc, a track jump or a configuration including a plurality of optical pickups is necessary in order to obtain information of an adjacent track. In the case where a track jump is performed for every access in order to obtain information of an adjacent track, a new problem occurs in that the transfer rate of a system does not improve. With the configuration including a plurality of optical pickups, the cost of the system increases.
Problem 4: When the track pitch is narrowed, there is not only an increase in the crosstalk amount for an RF signal but also a deterioration in an address signal for which a wobble is caused in a track with a predetermined method and added as address information. When the address signal deteriorates, acquisition of an address that identifies the position of an optical disc is difficult, and the access performance of the optical disc decreases. In the worst case, recording or reproduction for the optical disc is unfeasible. In the case where the address information has deteriorated due to crosstalk, it is difficult to identify the position for recording or reproduction. In the case of reproducing data from a recorded optical disc, it suffices to identify the reproduction position from an RF signal, since address information is superimposed on recorded data. However, in the case of recording data in an unrecorded optical disc, reproduction of an address signal is extremely important since an RF signal is not recorded. Particularly, in the case where an adjacent track is a recorded region, it is difficult to identify the position for recording. In Patent Literatures 4, 5, and 6, crosstalk cancel signal processing for an address signal is not described.